202 INVERTEBRATE PHYSIOLOGY 



organ discs in their new hosts. Likewise, the older discs in younger hosts 

 are retarded only until the equivalent host discs have reached the same 

 size. Once host and transplanted organs are in size harmony, their growth 

 continues alike ; this phenomenon is known as growth regulation. Now 

 the question arises : Why do the young discs ever catch up with the size of 

 the host discs, for the latter are, after all, growing in the same high hor- 

 mone titer ? The answer may be found in the physiological age of the discs. 

 The younger discs must be more responsive than the older ones to the 

 same hormone concentration. As the younger disc grows larger and be- 

 comes older, it must gradually lose its higher responsive capacity, until 

 host and graft tissue have the same reaction threshold. For the combina- 

 tion of older discs in younger hosts, the same holds true. The younger 

 host has a low titer of prothoracic-gland hormone — too low to support 

 much growth in the older disc, but still high enough to allow for consid- 

 erable growth of the responsive young organ disc. For the first time, then, 

 we have here an indication that the responsive capacity to humoral stimuli 

 changes with the age of an organ. At any time in development, the prevail- 

 ing hormone concentration and the responsive state of the target together 

 determine the growth velocity of an organ. This relationship underlies the 

 characteristic growth achievements which we call growth regulation. 



Additional information relating to responsive dififerences between young 

 and old issues is provided by the following observation. Drosophila has 

 three larval instars. When eye discs of young third-instar larvae are trans- 

 planted into late third-instar individuals, the transplant metamorphoses 

 prematurely in synchrony with the host, and gives rise to a small but other- 

 wise normal eye. Yet eyes from second-instar larvae transplanted in the 

 same manner are unable to metamorphose synchronously with the host 

 tissue. They continue to grow until they have reached the age at which 

 they can respond with difi^erentiation to the humoral factors (Bodenstein, 

 1939) . The essential point here is that very young organ discs can respond 

 to a certain prothoracic-gland hormone titer only with growth, while 

 older discs respond to the same titer with differentiation. The evidence 

 thus suggests that the young organ's response is a growth response, and 

 that the differentiation response is acquired later in development. As the 

 organ discs become older, they respond with differentiation to a hormone 

 titer which earlier elicited growth only. However, it must be understood 

 that a rather young organ, which in a low hormone titer responds with 

 growth, can be made to differentiate when the titer is raised. This is the 

 case in the above-mentioned experiment, where young third-instar discs 

 transplanted into older hosts differentiated prematurely. Other experi- 

 ments with similar implications will be discussed later in relation to 

 differentiation phenomena. 



